FIG. 4 is a simplified circuit diagram showing an integrated circuit 50 including a conventional bandgap reference circuit 51 for generating a bandgap reference voltage Vbg, a conventional thermal shutdown circuit 52 for generating a thermal shutdown control signal TSD, and a generalized functional circuit (e.g., a Power Management IC circuit) 53 that utilizes bandgap reference voltage Vbg during normal operation and is shutdown by thermal shutdown control signal TSD when an operating temperature of IC 50 equals or exceeds a predetermined maximum operating temperature.
Referring to the left side of FIG. 4, bandgap reference circuit 51 includes an operational amplifier (op amp) C1 that generates bandgap reference voltage Vbg at its output terminal. The non-inverting (+) input terminal of bandgap reference circuit 51 is connected between a resistor R1 and a diode Q1, which are connected in series between bandgap reference voltage Vbg and ground. The inverting (−) input terminal of bandgap reference circuit 51 is connected between resistors R2 and R3, which are connected in series with a diode Q2 between bandgap reference voltage Vbg and ground. Those skilled in the art recognize that bandgap reference circuit 51 represents only one of several possible circuit arrangements capable of generating the desired bandgap reference voltage Vbg. The particular circuit structure of bandgap reference circuit 51 is disclosed in Chapter 4, Appendix A4.3 of “Analysis and Design of Analog Integrated Circuits 2ND Edition”, Paul R. Gray and Robert G. Meyer (copyright 1977 by Wiley & Sons, Inc.).
In general, bandgap reference circuits function as temperature independent voltage reference circuits to provide a bandgap reference voltage Vbg at a voltage level typically around 1.25 V, which is close to the theoretical 1.22 eV bandgap of silicon at 0° K. In the example shown in FIG. 4, when power is supplied to IC 50, bandgap reference circuit 51 operates as follows. Assuming a stable operating point exists, then the differential input voltage of op amp C1 must be zero, and resistors R1 and R2 have equal voltages across them. Thus, the two currents I1 and I2 must have a ratio determined by the ratio of resistors R1 to R2. These two currents are the collector currents of the two diodes Q1 and Q2 (e.g., diode-connected transistors), assuming base currents are negligible. Thus, the difference between their base-emitter voltages can be represented by Equation 1:
                              Δ          ⁢                                          ⁢                      V            be                          =                                            V              T                        ⁢            ln            ⁢                                                            I                  1                                                  I                  2                                            ·                                                I                                      S                    ⁢                                                                                  ⁢                    2                                                                    I                                      S                    ⁢                                                                                  ⁢                    1                                                                                =                                    V              T                        ⁢            ln            ⁢                                                            R                  ⁢                                                                          ⁢                  2                                                  R                  ⁢                                                                          ⁢                  1                                            ·                                                I                                      S                    ⁢                                                                                  ⁢                    2                                                                    I                                      S                    ⁢                                                                                  ⁢                    1                                                                                                          Eq        .                                  ⁢        1            where VT is the threshold voltage across resistor R3, and IS1 and IS2 represent the saturation currents of diodes Q1 and Q2, respectively. The same current that flows through resistor R3 also flows through resistor R2, so the voltage across resistor R2 is represented by Equation 2:
                              V                      R            ⁢                                                  ⁢            2                          =                                                                              R                  ⁢                                                                          ⁢                  2                                                  R                  ⁢                                                                          ⁢                  3                                            ·              Δ                        ⁢                                                  ⁢                          V                              be                ⁢                                                                                                =                                                                      R                  ⁢                                                                          ⁢                  2                                                  R                  ⁢                                                                          ⁢                  3                                            ·                              V                T                                      ⁢            ln            ⁢                                                            R                  ⁢                                                                          ⁢                  2                                                  R                  ⁢                                                                          ⁢                  1                                            ·                                                I                                      S                    ⁢                                                                                  ⁢                    2                                                                    I                                      S                    ⁢                                                                                  ⁢                    1                                                                                                          Eq        .                                  ⁢        2            Note that Eq. 2 implies that the currents I1 and I2 are both proportional to temperature if the resistors have zero temperature coefficient. The output voltage of op amp C1 (i.e., bandgap reference voltage Vbg) is thus represented by Equation 3:
                              V          bg                =                                            V              be                        +                                                                                R                    ⁢                                                                                  ⁢                    2                                                        R                    ⁢                                                                                  ⁢                    3                                                  ·                                  V                  T                                            ⁢              ln              ⁢                                                                    R                    ⁢                                                                                  ⁢                    2                                                        R                    ⁢                                                                                  ⁢                    1                                                  ·                                                      I                                          S                      ⁢                                                                                          ⁢                      2                                                                            I                                          S                      ⁢                                                                                          ⁢                      1                                                                                                    =                                    V              be                        +                          KV              T                                                          Eq        .                                  ⁢        3            Eq. 3 implies that bandgap reference voltage Vbg is substantially independent of temperature, with the constant K set by the ratios of R2/R1, R2/R3 and IS2/IS1.
Referring again to FIG. 4, due to process variations and component mismatches typically encountered during the production of integrated circuits, bandgap reference circuit 51 is typically designed in a way that allows “trimming” of resistors R1, R2 and R3 at the end of the fabrication process in order to produce the desired bandgap reference voltage Vbg. This trimming process typically involves providing several resistors connected in series, and a mechanism (e.g., programmable elements) that can be used to bypass some of the resistors in order to supply accurate voltages to op amp C1. An exemplary trimmable resistor circuit is shown in FIG. 5, where resistors R21 to R24 are connected in series, with each node N21 to N23 between adjacent resistors connected to op amp C1 by way of programmable elements (e.g., fuses, anti-fuses or pass gates) P1 to P3. In this example, resistor R2 (see FIG. 4) is trimmed by selectively turning on zero or more of programmable elements P1 to P3. For example, the resistance of resistor R2 is minimized by closing programmable element P1 such that op amp C1 is coupled to bandgap reference voltage Vbg by way of node N21, whereby the total resistance of resistor R2 is equal to the resistance of resistor R21. Conversely, resistor R2 is maximized by leaving open all programmable elements P11 to P13 such that op amp C1 is coupled to bandgap reference voltage Vbg by way of node N24, whereby the total resistance of resistor R2 is equal to the sum of the resistances of resistors R21+R22+R23+R24.
Referring again to FIG. 4, one further common practice associated with the production of ICs including bandgap reference circuits is to provide a series of resistors RA to RD that are connected to bandgap reference voltage Vbg, and provide temperature independent reference voltage VREF1 to VREF3, which are tapped from node NAB, NBC and NCD, respectively. Reference voltages between Vbg and GND are easily generated for various PMIC functions.
Referring again to FIG. 4, thermal shutdown circuit 52 includes a thermal sensor 55 whose voltage is compared by an op amp 57 to a supplied reference voltage VREF, whereby thermal shutdown control signal TSD when the sensor voltage VSENSOR is equal to the reference voltage. Such thermal shutdown circuits are described, for example, in “Development of a 1 MHz MOSFET gate-driver for integrated converters”, M. A. de Rooij, J. T. Strydom and J. D. van Wyk, P. Beamer (IEEE publication 0-7803-7420-7/02 (2002)), in which the disclosed thermal sensor uses is a thermistor whose output signal is dedicated to the thermal shutdown of a functional circuit.
A problem associated with including thermal shutdown circuit 52 is that thermistor 55 and its associated circuitry are additional to any other thermally sensitive circuitry present on an IC having a bandgap reference circuit, and therefore take up a significant amount of valuable silicon area, consume a significant amount of power, and typically require a separate trimming operation (i.e., in addition to the trimming operation mentioned above with reference to bandgap reference circuit 51).
What is needed is a thermal shutdown circuit that minimizes silicon area and power consumption, and simplifies the trimming operations associated with its host IC.